This disclosure is related to a hydrostatic drive according to the description below.
Hydrostatic drives are used, in particular, as traction drives and can have a plurality of hydraulic motors in order to be able to drive a plurality of axles or a plurality of wheels individually.
In order to prevent a wheel slipping when there is deficient traction or in order to bring about synchronization of the rotational speeds of the individual wheels, it is known to connect hydraulic motors of two wheels in series, i.e. to connect the pressure medium outlet of the first hydraulic motor to the pressure medium inlet of the second hydraulic motor via a coupling line. If the swept volumes of the hydraulic motors are the same, the same rotational speed occurs at both hydraulic motors since both hydraulic motors deliver the same pressure medium flow.
If, for example, a wheel which is connected to the first hydraulic motor slips owing to deficient traction, the excess torque at this hydraulic motor is transmitted via the coupling line to the second hydraulic motor. The first hydraulic motor is, as a result, “supported” on the second hydraulic motor insofar as the traction at the wheel of the second hydraulic motor is sufficient for this. The torque is transmitted from the slipping wheel to the wheel with traction. With this solution, comparatively little driving torque is lost from the traction drive owing to the slipping of a wheel.
The series connection can be referred to as a lock. In the case of the transverse lock, the hydraulic motors of two wheels of one axle are connected in series, and in the case of the longitudinal lock the hydraulic motors of two wheels of a drive chain or of various axles are connected in series.
Drive concepts of hydraulic motors which are connected in series have a relatively poor efficiency level compared with hydraulic motors which are connected in parallel. A cause is relatively small pressure differences which occur at the hydraulic motors which are connected in series. High pressure is applied to the pressure medium inlet of a first, upstream hydraulic motor, and the pressure medium outlet of the downstream hydraulic motor is assigned to the low pressure. In the coupling line between these two hydraulic motors a pressure of medium level occurs, referred to as the “back pressure”. As a result, both hydraulic motors have a relatively small pressure difference between their pressure medium inlet and pressure medium outlet compared to the parallel connection, which leads to an overall lower total torque of the traction drive. The efficiency level of the drive is therefore reduced compared to the parallel connection.
Since severely degraded traction conditions occur only comparatively rarely, for example during offroad use or in the winter, it is advantageous generally to limit the drive energy to just one wheel or to just one axle, and to connect a further wheel or a further axle only in the case of deficient traction.
This is taken up by DE 10 2006 058 802 A1 in order to improve the total efficiency level, said document presenting a hydrostatic multi-motor drive with single hydraulic motors, and in which document hydraulic motors which are located downstream and are connected in series can be switched into the idling mode by means of a bypass line, a valve unit and a minimum pressure holding device. The minimum pressure holding device brings about here a comparatively low pressure at the pressure medium input of the downstream hydraulic motor in the idling mode, with the result that said hydraulic motor only has a low driving power which is of just such a magnitude that its self-locking is overcome. A disadvantage with this solution is that the efficiency level continues to be reduced in the four-wheel operating mode.
EP 0 547 947 B1 presents a hydrostatic traction drive having two axles on which in each case two wheels of hydraulic motors are driven. The wheels of the one axle are driven here in each case by a double hydraulic motor, and the wheels of the other axle by a single hydraulic motor. The double hydraulic motors are arranged hydraulically in parallel here, wherein one of the hydraulic motors of the double hydraulic motors is connected in series with a respective single hydraulic motor of the other axle. The double hydraulic motors each have two groups of hydrostatic working spaces which are mechanically coupled. Each of the groups has a pressure medium inlet and pressure medium outlet. As a result it is possible to divide the (swept) volume of the double hydraulic motors and actually connect one of these hydraulic motors hydraulically in series with a single hydraulic motor. When slip occurs at a wheel, traction of the traction drive is at least partially maintained owing to the hydraulically mutually supporting hydraulic motors. However, spinning of the wheels is possible if the pressure medium which drives the hydraulic motors flows out via pressure limiting valves when slip occurs.
EP 1 420 973 B1 discloses a hydrostatic traction drive in which four wheels are provided, wherein a respective wheel is driven with double hydraulic motors. A respective hydraulic motor of the double hydraulic motors is then connected in series with a respective other hydraulic motor of another double hydraulic motor.
A disadvantage with these solutions is that the arrangement and hydraulic connection of double hydraulic motors is extremely costly in terms of apparatus.
In contrast, the disclosure is based on the object of providing a hydrostatic drive which is of simple configuration in terms of apparatus, substantially prevents, as a traction drive, spinning of individual wheels or of a plurality of wheels, and at least partially maintains traction when individual wheels or a plurality of wheels spin. This object is achieved by a hydrostatic drive according to the features described below.